1. Field of the Invention
The present invention relates to a uninterruptive power supply for supplying uninterrupted A.C. power to a load.
2. Description of the Related Art
FIG. 7 is a circuit diagram, partly in block diagram, showing the configuration of a conventional uninterruptive power supply which is disclosed in Japanese Patent Application Laid-Open No. Hei 7-87686 for example.
In FIG. 7, reference numeral 1 designates an A.C. power source, reference numeral 2 designates a rectifier for rectifying an alternating current from the A.C. power source 1 into a direct current, reference numeral 3 designates a D.C. intermediate circuit which is connected to a D.C. output of the rectifier 2, reference numeral 4 designates a smoothing capacitor for smoothing the voltage developed across the D.C. intermediate circuit 3, reference numeral 5 designates an inverter for converting a direct current from the D.C. intermediate circuit 3 into an alternating circuit, and reference numeral 6 designates a load.
Reference numeral 7 designates a stepup/stepdown chopper for supplying the electric charges from the D.C. intermediate circuit 3 to batteries 8 and for discharging electric charge from the batteries 8 to the D.C. intermediate circuit 3. The stepup/stepdown chopper 7 includes the batteries 8 having the lower voltage than the voltage across the D.C. intermediate circuit 3, a smoothing capacitor 9, a chopper reactor 10, a first transistor 11, a first free wheel diode 12, a second transistor 13 and a second free wheel diode 14.
In addition, reference numeral 15 designates a chopper controlling circuit for operating the above-mentioned stepup/stepdown chopper 7 as the stepdown chopper during the non-interruption to charge the above-mentioned batteries 8, while for operating the above-mentioned stepup/stepdown chopper 7 as the boosting chopper during the detection of the service interruption to supply the direct current to the above-mentioned D.C. intermediate circuit 3 with the above-mentioned batteries 8 as the D.C. power source. The chopper controlling circuit 15 includes a first gate driving circuit 16, a second gate driving circuit 17, and a pulse generating circuit 18 for outputting the gate pulse to the first gate driving circuit 16 and the second gate driving circuit 17 depending on whether or not the service interruption has been detected. In addition, reference numeral 19 designates a voltage sensor for detecting the voltage of the A.C. power source 1, and reference numeral 20 designates a service interruption detector for detecting the service interruption of the A.C. power source 1 on the basis of the output of the voltage sensor 19.
Next, the operation will hereinbelow be described.
Normally, the rectifier 2 rectifies the alternating current from the A.C. power source 1 into the direct current to output the resultant direct current to the D.C. intermediate circuit 3. The inverter 5 converts the direct current from the D.C. intermediate circuit 3 into the alternating current to apply the resultant A.C. output to the load 6.
At this time, if the service interruption detector 20 for detecting the service interruption of the A.C. power source 1 has not detected the service interruption on the basis of the output from the voltage sensor 19 for detecting the voltage of the A.C. power source 1, then the pulse generating circuit 18 outputs the pulse to the first gate driving circuit 16 in such a way that the first transistor 11 carries out repeatedly the ON/OFF operation.
That is, at the time when the pulse generating circuit 18 has operated the first gate driving circuit 16 to turn ON the first transistor 11 on the basis of the gate signal from the first gate driving circuit 16, the charging current is caused to flow into the batteries 8 through the path of the D.C. intermediate circuit 3.fwdarw.the first transistor 11 the chopper reactor 10.fwdarw.the batteries 8.fwdarw.the D.C. intermediate circuit 3. Next, at the time when the first transistor 11 has been turned OFF, the current which has been caused to flow through the chopper reactor 10 circulates through the path of the chopper reactor 10.fwdarw.the batteries 8.fwdarw.the second free wheel diode 14.fwdarw.the chopper reactor 10 so that the stepup/stepdown chopper 7 operates as the well known stepdown chopper including the first transistor 11, the chopper reactor 10, and the second free wheel diode 14 with the D.C. intermediate circuit 3 as the D.C. power source to charge the batteries 8.
On the other hand, at the time when the service of the A.C. power source 1 has been interrupted, the service interruption detector 20 outputs the signal to the pulse generating circuit 18 which outputs, in turn, the pulse to the second gate driving circuit 17 in such a way that the second transistor 13 carries out repeatedly the ON/OFF operation.
That is, at the time when the pulse generating circuit 18 has operated the second gate driving circuit 17 to turn ON the second transistor 13 on the basis of the gate signal from the second gate driving circuit 17, the current is increasingly caused to flow through the path of the batteries 8.fwdarw.the chopper reactor 10.fwdarw.the second transistor 13.fwdarw.the batteries 8 with the batteries 8 as the power source. Next, at the time when the second transistor 13 is turned OFF, the current is caused to flow through the path of the chopper reactor 10.fwdarw.the first free wheel diode 12.fwdarw.the D.C. intermediate circuit 3.fwdarw.the inverter 5 and hence the stepup/stepdown chopper 7 operates as the well known boosting chopper including the second transistor 13, the chopper reactor 10, and the first free wheel diode 12 with the batteries 8 as the D.C. power source to supply the direct current to the D.C. intermediate circuit 3 so that the inverter 5 supplies the alternating current to the load 6 in the uninterruptive manner.
Since the conventional uninterruptive power supply is configured as described above, the power source for supplying the load 6 with the electric power is either the A.C. power source 1 in the normal case, or the batteries 8 when the service interruption occurs. Therefore, there arises the problem in that the power consumption of the load 6 is increased, and hence when exceeding the supply ability of the A.C. power source 1 or adjusting the demand electric power on the A.C. power source 1 side, the electric power supplied from the A.C. power source 1, i.e., the input of the rectifier 2 can not be limited.